The majority of digital imaging appliances use a Charge Coupled Device (CCD) array or a Complementary Metal-Oxide-Semiconductor (CMOS) sensor to capture a digital image. In order to properly construct an image light from the object being imaged must be properly focused onto the array or sensor. In auto focusing digital imaging appliances, this is accomplished by acquiring multiple images with different focus settings. A focus figure-of-merit is calculated for each of these images. The figures-of-merit are then used by an algorithm to select the focus setting which focuses the image on the array or sensor.
One popular method to focus a camera is to drive the lens through a number of focus positions and capture a focus image at each of them (typically between 5 and 20 such frames are captured). A focus metric is then accumulated over one or more chosen regions of the focus images, typically called focus zones, based on various image filters which respond to suitable parts of the frequency spectrum, either band-pass or high-pass, and suppress noise. The lens position which corresponds to a peak, measured or interpolated, for one or more zones is then chosen to represent true focus.
However as this process takes time and for a hand held device or moving scene it is possible for motion to corrupt the focus metric. Within frame motion artifacts can be mitigated by limiting the focus exposure to a predetermined threshold, typically 1/60th of a second. However between frames the scene region corresponding to each zone can change resulting in a change of the metric that is not related solely to focus.
A difficulty is that during the auto focusing, the camera can move slightly or objects that are being imaged can move between the acquisition of different frames. In order for cameras to properly focus, this motion needs to be accounted for when calculating the figure figure-of-merit for each of the images. Motion estimation has been used to track the individual zones and compute the focus metric over the tracked zone location. This approach is often successful but is highly complex and therefore not compatible with typical focus support hardware and is thus only feasible if it can be implemented in firmware or as part of a custom hardware solution. Such solutions are expensive and tend to consume large amounts of power which is not desirable for consumer appliances such as mobile phone camera modules.
The adjustment of the figure-of-merit to compensate for scene movement is known from U.S. Pat. No. 7,428,375 B2.